In plastic injection molding processes, a thermoplastic or thermoset molding compound is first heated to plasticity in an injection cylinder at controlled temperature. Afterwards, the plasticized compound is forced from the cylinder through a nozzle by means of pressure generated within the cylinder. On emerging from the nozzle, the plasticized compound is conveyed through a hole in a mold plate, usually stationary, by means of a sprue bushing (sprue for short) into the runner system or gate of the mold cavity. Once in the cavity, the resin assumes the shape of the cavity and is then cooled to the point where it is sufficiently solidified to retain the desired cavity shape. The mold is then opened, and the part is ejected or otherwise removed. The entire process is usually automated with the clamping of the mold parts prior to injection, and unclamping for part ejection or removal after cooling, taking place under the control of a microprocessor or other form of automated controller. Machines are rated by the number of ounces they will inject per piston or screw stroke and by the square inches of working area that can be clamped against injection pressure. Consequently, a machine can be used to fabricate a variety of different sized parts up to its maximum capacity. This is accomplished by using, among other things, plates of suitable thickness for the part at hand; large working areas and high injection pressures calling for robust mold plates to mount and sustain the large stresses created during the molding cycle. Plates used to make small parts, on the other hand, can be scaled down in size in accordance with the reduced stresses generated while molding them. Parts between the largest and smallest obviously require intermediate sized plates, and the industry has adopted standard plate thicknesses to promote productivity.
For best process performance regardless of part size, it is known to be beneficial to keep the temperature of the plasticized compound more or less constant throughout its travel to the cavity. This reduces process problems associated with material degradation due to thermal variability, improves yield by decreasing scrap losses, and increases machine production time by reducing down time due to freeze offs.
However, while standardized in many respects, particularly with respect to mold base or plate thicknesses, present molding machinery does not always provide for precise temperature control to take advantage of its beneficial effects. Indeed, much of the available machinery is still run employing cold sprue bushings which allow the temperature of the resin to be poorly controlled from the time it leaves the nozzle until it reaches a zone in the mold where temperature control is reacquired with, for example, internal heating channels in the mold.
More sophisticated practice, apparently not yet universally accepted, recognizes the advantages of controlling temperature by employing hot sprue bushings to convey material from the nozzle to the cavity gate, often times through the fixed mold plate, sometimes referred to as the “A” plate or base. A variety of approaches for providing heat in these hot sprue bushings have been used. Among these are the use of resistive heating elements and heat pipes such as those described in U.S. Pat. No. 4,034,952 entitled “HOT PLASTIC INJECTION BUSHING” issued on Jul. 12, 1977. In the latter case, the heat pipes are used to transfer heat from electrically powered heater bands located at the nozzle end of the sprue bushing to regions along the bore near the tip.
U.S. Pat. No. 5,213,824 entitled “ADJUSTABLE HOT SPRUE BUSHING” issued on May 25, 1993, and assigned to the same assignee as the present application, describes a hot sprue bushing of adjustable drop length so that it can be used in injection molding machines with a full range of different mold plate thicknesses to transport plasticized material from injection nozzles to mold cavities while controlling the temperature of the plasticized material. This hot sprue bushing comprises an elongate body having a head, a stem and a tip; the head has a reference seating surface which faces the tip. The elongate body has a through bore passing from end to end through the head, stem and tip of the body for transporting plasticized material through the sprue bushing from the head to the tip thereof. The body also has at least one non-through bore extending alongside the through bore, in heat conducting relationship with respect thereto; the non-through bore extending from the head through the stem and into the tip but is shorter than the elongate body. The, or each, non-through bore receives an electrically powered heating cartridge for controlling the temperature of the plasticized material as it travels along the through bore. In a preferred form of this hot sprue bushing, the body contains yet another bore which receives and positions a thermocouple substantially at the tip of the elongate body for controlling the temperature of the through bore over its full length.
Adjustment of the drop length of the hot sprue bushing is effected by means of a changing means manually positionable against the reference seating surface of the head and between this reference seating surface and the tip for shortening the initial drop length of the elongate body. This changing means has a mold plate seating surface selectively locatable over a predetermined range of distances between the reference seating surface and the tip to change the drop length of the hot sprue bushing, the changed drop length of the hot sprue bushing corresponding to the distance between the mold plate seating surface of the changing means and the tip. The hot sprue bushing may also have a cap which removably attaches to the head and has a recess having a shape complementary to that of standard injection nozzle shapes.
The adjustable hot sprue bushing of U.S. Pat. No. 5,213,824 gives good control of the temperature of the plasticized materials passing therethrough and is in commercial use. However, because the body of this hot sprue bushing needs to accommodate the through bore for the plasticized material, one or more bores, one for each heater cartridge, and the bore for the thermocouple, the body needs to be of substantial cross-section, and thus has a high thermal mass. This high thermal mass requires substantial heat input from the heater cartridges to maintain the proper operating temperature in commercial use, and thus the heater cartridges are subjected to a heavy-duty cycle, which reduces the working life of the heater cartridges. Such cartridge failure results in substantial down time of the of the injection molding machine, since it is necessary to disassemble the hot sprue bushing in order to replace the failed heater cartridge. It has also been found that, in commercial use, the cap of the hot sprue bushing is easily damaged and detached from the head. Finally, in the hot sprue bushing of U.S. Pat. No. 5,213,824, the wiring for the heater cartridges emerges from the bushing on a bracket secured to the cap of the bushing, and thus immediately adjacent the injection nozzle, in which location the wiring is somewhat susceptible to damage.
U.S. Pat. No. 6,095,789 entitled “ADJUSTABLE HOT SPRUE BUSHING” issued on Aug. 1, 2000, and assigned to the same assignee as the present application, describes a hot sprue bushing comprising an elongate body having a head, a stem and a distal end portion, and a through passage extending from end to end through the head, stem and distal end portion of the body for transporting plasticized material from the heat to the distal end portion. The distal end portion bears a first engaging device, and the head is of greater cross-section than the stem so that the head provides a first abutment surface extending outwardly of the stem. An electrically powered heater is wrapped around the stem of the head in heating conducting relationship. A sleeve surrounds the electrically powered heater; and a tip member has a second engaging device engaged with the first engaging device on the distal end portion, the tip member having a second abutment surface engaged with one end of the sleeve so as to urge the opposed end of the sleeve into engagement with the first abutment surface on the head. The tip member permits egress of plasticized material from the through passage in the body.
The hot sprue bushing of U.S. Pat. No. 6,095,789 provides control of the temperature of the plasticized materials passing therethrough, and has a desirably lower thermal mass so that the heater of the bushing can be operated at a low duty cycle, and thus with an improved working lifetime, as compared with the heater cartridges used in U.S. Pat. No. 5,213,824. Also, the hot sprue bushing of U.S. Pat. No. 6,095,789 permits location of a thermocouple adjacent the tip of the body for controlling the temperature of the through bore over its full length, while still allowing adjustment of the drop length of the hot sprue bushing, and allowing the wiring for the hot sprue bushing to emerge from the injection molding machine at a point spaced from the bushing, thereby rendering this wiring less susceptible to damage.
However, there is a trend in the injection molding industry towards the use of multiple bushing assemblies comprising a large number of hot sprue bushings fed from a single source of heated plastic material. In such multiple bushing assemblies, it is desirable that keep the cross-section of the individual bushings as small as possible in order to accommodate the maximum number of bushings within an assembly of given size and/or to provide maximum freedom in varying the number of bushings in the assembly. Since one broken or malfunctioning bushing may also render the entire assembly unusable, it is important that individual bushings can be removed and accurately relocated within the assembly. Since the tips of the bushings are subject to the most rapid wear, the bushing should provide for rapid removal and replacement of the tips. The individual bushings should be adapted to be fed from the single source of heated plastic material without requiring excessive lengths of piping, and in practice the easiest way to arrange this is to have the individual bushings fed from spaced lines of ports through which the plastic material is supplied.
Viewed against these criteria regarding suitability for use in multiple bushing assemblies, the hot sprue bushing of U.S. Pat. No. 6,095,789 has certain disadvantages. One disadvantage is the rather large cross-section of the bushing relative to the cross-section of the body. In the specific embodiment shown in the drawings of this patent, the length adjusting member has the form of a hollow cylinder having an internal thread which engages an external thread on the body of the bushing. The diameter of the length adjusting member is approximately twice that of the body, which requires a larger than desirable spacing between adjacent bushings in a multiple bushing assembly. Replacement of the tip of the bushing requires removal of the tip member, which in turn releases the sleeve and thus removes the compression on the heater. Thus, after replacement of the tip, the bushing must be carefully reassembled to ensure that the heater is not accidentally damaged. Finally, because adjustment of the drop length of the bushing is effected by moving the whole body relative to the length adjusting member, varying the drop length of the bushing also varies the position of the inlet end of the through passage relative to the surface on which the length adjusting member is mounted. If such a bushing is used in a multiple bushing assembly and it is desired to use different drop lengths for different bushings of the assembly (as is commonly the case where the assembly is used to make a plurality of different types of moldings simultaneously), the varying positions of the inlet ends of the through passages lead to problems in arranging the piping to supply heated plastic material to these inlet ends; either inconvenient flexible piping must be used or equally inconvenient arrangements must be made to allow for variation of the positions of the nozzles used to supply material to the inlet ends.
It is an object of the present invention to provide a hot sprue bushing which retains the ability of the hot sprue bushing of U.S. Pat. No. 6,095,789 to supply heat to the plastic material over essentially the full length of the bushing, but which is better adapted for use in multiple bushing assemblies.
It is a further object of the present invention to provide a hot sprue bushing which can readily be removed and replaced in a multiple bushing assembly, with minimal risk of damage to the heating element of the bushing.
It is a further object of the present invention to provide a hot sprue bushing which permits multiple bushings to be fed in a convenient manner from a single source of heated plastic material.
Other objects of the invention will be apparent and will appear hereinafter in the following detailed description when read in connection with the drawings.